This invention provides a multi-purpose package technology which suits the requirements of cryogenically cooled electron devices over the entire 10 to 150 K range, aiming at meeting standards of operational performance and lifetime reliability comparable to those set for devices operating at room temperature and suitable for mass production.
Low temperature operation requires the electron device component be encapsulated in an evacuated container. Such a vacuum encapsulation is commonly designated as DEWAR. The package of the invention is of the dewar type, and the innovation concerns the cryogenic interface between the cooler and its thermal payload, the encapsulated electron device.
The cryogenic interface is constituted of a coldfinger member onto which the electron device is mounted in the inner evacuated portion of the encapsulation. The non-evacuated portion of the coldfinger is mechanically connected to a cryocooler. The cold tip of the cooler is in thermal contact with a small area of the wall of the coldfinger close to the device, thereby causing heat from the device in the interior to flow through a multi-layer heatpath comprising the wall of the coldfinger, into the exterior cryogenic heat sink.
It is well known to those skilled in the design and manufacture of cryogenically cooled electron devices that the multi-layer heatpath involves a particularly complex problem, namely, how to get the cold into the active electronic component most effectively, while at the same time ensuring a vacuum life performance appropriate to modern package standards.
Many different Dewar versions have been proposed prior to the invention.
An illustrative example is a dewar for multielement infrared detectors (see U.S. Pat. No. 4,206,357). Useful as it is for the specific applications in thermal imaging, it however was severe limitations regarding its use for other applications: the cryogenic interface provides borosilicate glass as coldfinger wall material. Glasses have high thermal resistivities, and the temperature drop across the glass layer limits the use of the dewar to devices with low heat payload and limited physical dimensions, or to those in which temperature non-uniformity or floating are acceptable.
Another similar dewar, see U.S. Pat. No. 4,059,764, provides Kovar as coldfinger wall material which is a much better heat conductor than glass.
However, in both references, the heatpath of the cryogenic interface comprises at least one layer of an organic resin such as epoxy between the coldfinger wall and the electron device. Expoxies are very poor heat conductors with thermal resistivities about ten times higher than even that of glass. The use of Kovar is therefore an improvement compared with the first version but it is not a solution for the problem of constructing a multi-purpose dewar package which suits thermal requirements of the entire class of the aforementioned advanced electron devices.
It is also known that epoxies are long-term degassing sources and are inconsistent with the requirements for reliable, hard vacuum devices: Unpredictable virtual leak rates and total amount of released gas due to minute variations in resin composition from lot to lot make this kind of material class highly detrimental for dewar mass production.